An electrical device for current conditioning is known from the U.S. Pat. No. 5,379,020. Examples of current conditioning devices are also sometimes referred to as current limiters or fault current limiters. In U.S. Pat. No. 5,379,020, the current conditioning is provided by a electrical device consisting of a primary spool, a secondary spool which contains a quenchable superconductor exhibiting a transition from a low resistive state, commonly referred to as the superconducting state, to a high resistive state when a critical value of the electric current is exceeded. The secondary spool is coupled through a common part of magnetic flux with the primary spool. The secondary spool further comprises a metallic member which forms a closed loop circuit. The secondary spool is positioned in a cryostat that provides a cooling of the secondary spool. Magnetic coupling of two spools is provided though a ferromagnetic core.
A fault current limiting device is also known from the U.S. Pat. No. 5,694,279. The device comprises a primary spool comprising a metal or alloy and a secondary spool which includes quenchable superconductor which exhibits a transition from the low resistive state to the high resistive state when a critical value of the electric current is exceeded. Both of these spools are coupled through a common part of magnetic flux provided in a ferromagnetic core. The secondary spool is based on a number of thin and flat discs substrates coated with a layer of the quenchable superconductor, namely, a high temperature superconductor. The discs are provided with central openings which allow to install the discs in a cryostat and to position them around a magnetic core. The secondary spool may also comprise a cylindrical substrate coated with quenchable, high temperature superconductor.
Both of the above-referenced devices aim to condition electrical current in an external circuit which is connected in series to the primary spool. They may provide the function of a current limiter which provides limitation of the “primary” overcurrents. However, conventional devices display a reaction time which is too slow for some applications and may be too slow for efficient control of electric power.